1. Field of the Invention
The present invention relates in general to polishing of materials, and relates in particular to a method of determining an endpoint in a polishing process to provide a flat mirror polished surface on objects having fine internal structures such as semiconductor wafers.
2. Description of the Related Art
High density integrated semiconductor devices of recent years require increasingly finer microcircuits, and the interline spacing has also shown a steadily decreasing trend. For optical lithography operations based on less than 0.5 micrometer interline spacing, the depth of focus is shallow and high precision in flatness is required on the polishing object which has to be coincident with the focusing plane of the stepper.
Therefore, it is necessary to make the surface of a semiconductor wafer flat before fine circuit interconnections are formed thereon. According to one customary process, semiconductor wafers are polished to a flat finish by a polishing apparatus.
One conventional polishing apparatus comprises a turntable with a polishing cloth attached to its upper surface and a top ring disposed in confronting relationship to the upper surface of the turntable, the turntable and the top ring being rotatable at respective independent speeds. The top ring is pressed against the turntable to impart a certain pressure to an object which is interposed between the polishing cloth and the top ring. While an abrasive liquid containing abrasive material is supplied onto the upper surface of the polishing cloth, the surface of the object is polished to a flat mirror finish by the polishing cloth which has the abrasive material thereon, during relative rotation of the top ring and the turntable.
A device for detecting an endpoint of the polishing process which is used in the conventional polishing apparatus is disclosed in, for example, a U.S. Pat. No. 5,036,015. In the U.S. Pat. No. 5,036,015, a wafer to be polished is a multilayer material comprising a semiconductor layer, a conductor layer and an insulator layer. The frictional force between the polishing cloth and the wafer changes during a polishing process, as a surface layer is removed and an underlayer of the surface layer becomes exposed. According to this method, an endpoint is detected when a different underlayer becomes exposed.
A change in the frictional force is detected as follows. The wafer is polished at some distance away from the center of rotation of the turntable so that the point of application of the frictional force is eccentric, and this eccentricity causes a torque load on the turntable. When the turntable is driven with an electric motor, the torque can be measured as a function of the current flowing through the motor. Therefore, by monitoring the current, and suitably processing the resulting signal, it is possible to detect an endpoint as a change in the current measured.
In this type of conventional polishing apparatus, the top, ring holding the wafer is oscillated on the polishing cloth, in addition to the rotational motion of the top ring. The purpose of oscillation of the top ring is not only to prevent local wear of the polishing cloth and prolong the service life of the polishing cloth but also to prevent degradation in the flatness of the wafer caused by localized use of the polishing cloth.
However, such oscillating motions present a problem in detecting an endpoint from measurements of changes in the torque. This is because the point of application of the frictional force changes as the top ring is oscillated, and thus the torque applied to the turntable changes with the point of application of the frictional force. That is, since the torque is represented as a product of a frictional force and a distance from a center of the turntable to the point of application of the frictional force, the torque is affected by the change of the distance. Therefore, even if the torque is detected, the frictional force cannot be determined.